This invention relates to semiconductor devices and, more specifically, to a device which can be used as an improved transistor. The invention has particular application as a so-called modulation-doped field-effect transistor (MODFET).
The GaAs/AlGaAs MODFET has become well recognized for its superior low-noise performance compared with the GaAs MESFET. However, cryogenic operation of this device, where its advantages over the GaAs MESFET are most apparent, has not been ideal. So-called deep levels ("DX centres") in the Al.sub.x Ga.sub.1-x As with peculiar persistent properties can lead to the "collapse" of I/V drain characteristics. (See Drummond et al., "Bias Dependence and Light Sensitivity of (AlGa)As/GaAs MODFETs at 77K", IEEE Trans., 1983, pp. 1806-1811.) Also troublesome is the threshold voltage shift which occurs at 77K after the gate has been forward biased.
It has been suggested that one way to avoid these problems is to use Al.sub.x Ga.sub.1-x As with x.ltoreq.0.2, where the DX occupation probability is significantly reduced. (See Lang et al., "Trapping Characteristics and A Donor-Complex (DX) Model For A Persistent Photoconductivity Trapping Center in Te-Doped Al.sub.x Ga.sub.1-x As", Phys. Rev. B, 1979, 19, pp. 1015-1020.) Using a reduced Al mole fraction, however, reduces the conduction-band discontinuity of the heterointerface, which results in less efficient electron transfer and therefore a smaller two-dimensional electron gas (2DEG) concentration. Use of low Al mole fraction AlGaAs also compounds problems of the parasitic MESFET effect. (See Lee et al., "Parasitic MESFET in (Al,Ga)As/GaAs Modulation Doped FETs and MODFET Characterization", IEEE Trans., 1984, ED-31, pp. 29-35.)
An alternative solution proposed recently which avoids the AlGaAs altogether is to use InGaAs as the narrow bandgap material and GaAs as the larger-bandgap material. (See Rosenberg et al., "An InGaAs/GaAs Pseudomorphic Single Quantum Well High Electron Mobility Transistor", IEEE Electron Device Letters, EDL-6, 10, 1985.) In this structure, a quantum well (200 Angstroms in the device set forth) of In.sub.0.15 Ga.sub.0.85 As was sandwiched between confining GaAs layers, the upper of which was silicon-doped. The In.sub.0.15 Ga.sub.0.85 As quantum well layer is a strained layer (i.e., mismatch is accommodated as elastic strain). However, the use of GaAs as the high bandgap material has a similar drawback to the one noted above with respect to the small Al mole fraction in Al.sub.x Ga.sub.1-x As; namely, that the conduction-band discontinuity is too small to achieve an adequate 2DEG concentration.
It is among the objects of the present invention to provide a semiconductor device which exhibits, inter alia, improved operating characteristics, and which overcomes problems of prior art devices as just set forth.